A pair of 3D movie glasses with the lenses punched out have been frequently thrown into the mix, creating a frisson to a wide range of play scenarios.
The crib mattress has an even higher status. It has been a trampoline, multiple (multiple!) versions of slides and launchers of all sorts. It has a place of honour/hiding in our living room from which it is extracted atleast once a day. It fills all kinds of gaps.
I remember a moment in time when giving my kid a spoon in a restaurant got us through a crucial 15 minute interval.
Loose parts make me have a soft spot of puzzle pieces that have lost their tribe, car parts that have been abandoned to live another day as another part of another toy and even single Barbie shoes. Their "looseness" sets my kids free to figure out where they can be re-attached.